Gas turbines are widely used in commercial operations for power generation. A gas turbine typically includes a compressor located at the front, one or more combustors around the middle, and a turbine at the rear. The compressor can include multiple stages of compressor blades attached to a rotor. Ambient air, as a working fluid, enters an inlet of the compressor, and rotation of the compressor blades progressively compresses the working fluid. The compressor can include inlet guide vanes (IGVs) and variable stator vanes (VSVs) which can be used to control the flow of ambient air into the compressor.
Some of the compressed working fluid is extracted from the compressor through extraction ports for other use, and the remainder of the working fluid exits the compressor and flows to the combustors. The working fluid mixes with fuel in the combustors, and the mixture ignites to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases exit the combustors and flow to the turbine where they expand to produce work.
During the design cycle, a new compressor rig design typically must be validation tested and mapped to determine whether the compressor rig design will achieve critical to quality standards. For example, various tests can be performed to determine compressor airfoil steady state and transient aeromechanics. As another example, compressor maps can be generated for the compressor at various different shaft speed and load conditions. The compressor maps can be used to determine surge margins for the compressor rig as well as the aerodynamic design point for the compressor.
Due to the high shaft power demand, it is often difficult to perform validation testing and mapping over the full operating range of a full scale compressor rig. For instance, a gas turbine does not become self-sustaining until it achieves a relatively high percentage of full shaft speed. Thus, it can be difficult to obtain partial speed mapping of a compressor rig or gas turbine because the gas turbine would either operate outside of its operability range or below the self-sustaining speed. In addition, a gas turbine often cannot contribute enough starting power to start a compressor rig.
Validation testing and mapping of a compressor rig are often performed using sub-scale compressor rigs. Such sub-scale compressor rigs may range from ⅓ to ⅕ the scale of a full scale compressor rig. The design of a sub-scale compressor rig usually has to be carefully managed so that airfoil attachment and associated damping effects are not compromised. The testing of sub-scale compressor rigs can also create a need to correct performance measurements to address, for instance, Reynold's number effects, blade tip clearance, and thermal growth differences. Furthermore, the design of a sub-scale rig usually precedes the design of the full scale counterpart, requiring dedicated resources for design, product definition and procurement. Indeed, the need to design a sub-scale rig can add about two years to the overall design cycle of a new product.
Various techniques are also known for testing a gas turbine. For instance, it is known to use a water brake or other load device to simulate various load conditions for a gas turbine during gas turbine testing. However, such testing is limited for high power mapping because of the increased fuel flow requirements and because the turbine temperatures can exceed the limits of the turbine hardware. Partial speed mapping using this technique also may not be achievable because the gas turbine would either operate outside its operability range or below the self-sustaining speed.
Gas turbines and compressor rigs can be validation tested and mapped at a power generation site while connected to the power grid. However, off frequency (partial speed) testing of a gas turbine or compressor rig cannot be performed while connected to the power grid. Moreover, testing of a gas turbine or compressor rig at the power generation site often results in inconvenience to the power generation provider and imposes limits on the ability to redesign should the validation testing or compressor mapping reveal a potential problem with the gas turbine or compressor rig.
Thus, there is a need for an apparatus and method that can be used to test a full scale compressor rig over the entire speed and load operating range, allowing for full compressor mapping from choke to stall at full load, part load (power turn down) and partial speed conditions. There is also a need for an apparatus and method that can be used to test a compressor rig or gas turbine over the full range of operability for the compressor rig or gas turbine without having to connect the gas turbine and compressor rig to the power grid at the power generation site.